1. Field of the Invention
The present invention relates to an improvement of a wire electrode used for electro-discharge machining.
2. Description of the Background Art
Wire electro-discharge machining is a method of melting and cutting a workpiece into a desired configuration by causing intermittent discharge between a workpiece and a linear working electrode referred to as a wire electrode for electro-discharge machining via a working liquid such as water or oil and by moving the workpiece relative to the wire electrode for electro-discharge machining. This method is utilized for manufacturing various types of metal molds and the like. Such wire electro-discharge machining requires processing characteristics such as excellent finish of a workpiece, high process accuracy, good finished surface condition of the workpiece, no wire electrode material adhesion to the workpiece, and a short electro-discharge machining time. A brass wire, having good drawability and high strength required for a wire electrode, is generally used as a wire electrode for such wire electro-discharge machining.
In recent years, as improvements and advancements are made on working power sources, a need exists for a wire electrode that can improve processing speed and accuracy. In particular, a wire electrode that allows improved processing speed and accuracy is desired for applications employing a power source of a wire electro-discharge machining apparatus that repeatedly applies a high and short-time pulse voltage.
Conventional wire electrodes for electro-discharge machining prepared from a brass wire, however, are incapable of attaining a sufficiently high processing speed. Further, brass wires are problematic in that large amount of electrode material adheres to the workpiece, that the cut surface of the workpiece is roughened, and that the wire electrode is easily broken when electro-discharge machining is attempted at a high processing speed.
Moreover, some applications use a wire electrode for wire electro-discharge machining prepared by covering a core formed of Cu or a Cu alloy with Zn, or that, after covering with Zn, heat treated to produce a Cu--Zn alloy at a surface layer by diffusion and thus having an oxide film on the outermost surface. The former Zn-coated Cu or Cu alloy wire electrode results in improved cut surface of the workpiece; however, it cannot provide a sufficiently high processing speed. On the other hand, the latter diffusion alloy-coated wire electrode improves the processing speed to some extent, but the property of the cut surface of the workpiece is not sufficiently improved. It is also difficult to position the latter wire electrode prior to the start of electro-discharge machining. As compared with the brass wire, although the conventional wire electrode for wire electro-discharge machining having a core and a coating layer formed on an outer periphery of the core affords improved electro-discharge machining characteristics, the wire electrode itself, a wire contact (a roller or die for feeding electricity), a guide die and the like suffer wear and undergo considerable damage so that their lifetimes are shortened, which significantly increases the cost of electro-discharge machining.